As disclosed in e.g. publication U.S. Pat. No. 6,267,900 a solvent extraction settler arrangement is known which is adapted for hydrometallurgical liquid-liquid extraction processes and which typically comprises a mixing unit for preparing a dispersion from mutually immiscible solutions which include a heavy solution phase and a light solution phase. A settler tank is arranged to separate solution phases from a dispersion while the dispersion fed from the feed end flows to the discharge end of the settler tank. Elongated discharge launders are arranged at the discharge end of the settler to receive from the settler and discharge each solution phase that is separated from the dispersion in the settler. Each discharge launder includes a first end to which an outlet box is arranged and a closed second end.
One discharge launder is arranged at the discharge end of the settler for collecting and discharging the heavy solution phase which is separated as an underflow from the settler tank. The discharge launder includes a first end, a first outlet arranged at the first end, and a closed second end. An outlet box is connected to the first end of the discharge launder to receive the heavy solution phase flowing via the first outlet from the discharge launder. It may occur that on the surface of the heavy solution phase there is still a layer of entrained light solution phase. The outlet box comprises a vertical shaft defined by a side wall, a bottom and a top wall. Adjacent the upper part of the shaft there is an inlet or receiving the heavy solution phase together with said layer of entrained light solution phase flowing from the first outlet of the discharge launder. The outlet box also comprises a discharge outlet for discharging the heavy solution phase from the outlet box. The discharge outlet is located below the level of the inlet.
The entrained light solution phase (which is normally the organic phase) on the surface of the heavy solution phase (which is normally the aqueous phase) causes problems, if it is allowed to enter the next stage of the process. In a multi-stage solvent extraction process the most important process stage the outlet flow of which should not include any entrained organic phase impurities is the last extraction phase from where the outlet flow of the raffinate goes to a raffinate pond. Another process stage the outlet flow of which should not include entrained organic phase impurities is the stripping stage from where the outlet flow goes to the rich electrolyte tank to store the enriched electrolyte to be further lead to electrowinning. The entrained organic remains going further in the process also means additional costs because reagent losses must be replaced.